This invention relates to a holography apparatus for evaluating the size, shape and position of cracks present in the structure such as in the piping arrangement of an atomic reactor, for example, by the use of digital ultrasonic hologram.
The conventional ultrasonic holography apparatus obtains the information on cracks in an object by irradiating an ultrasonic pulse (transmission wave) of a sine wave mode from a transducer, receiving a reflected wave or transmission wave from the object (hereinafter referred to simply as "object modified wave"), causing the object modified wave to interfere with a reference wave having a predetermined phase difference from the transmission wave to obtain an interference wave, and luminance-modifying the amplitude of the interference wave so obtained to form an ultrasonic hologram of the object.
The apparatus of the above-described type is characterized in that it uses electric signals for the transmission wave and the reference wave so as to control the phase difference between them by an electric circuit and that the amplitude of the interference wave can be obtained by multiplication of the electric signals.
However, the conventional apparatus involves the following drawbacks. For one thing, since the transmission wave is caused to interfere with the reference wave, from several to dozens of seconds of pulse width of the received wave is required whereby the reflected waves from plural objects can not easily be discriminated with respect to time if the plural objects are present adjacent to each other. For another thing, since a ultrasonic pulse beam of a sine wave mode is transmitted from the transducer, the size of a receiver as well as a power amplifier become inevitably greater whereby maneuverability of the apparatus is lowered correspondingly. In addition, the gap of interference fringes of the resulting hologram is restricted depending on the ultrasonic frequency used.